TECHNICAL FIELD The present invention relates to a specific polyester resin composition containing a hydroxy carboxylic acid. More particularly, the invention relates to a hydroxy carboxylic acid-containing polyester resin composition which has a specific bonding relation between a hydroxy carboxylic acid unit and its neighboring units and which is excellent in gas barrier properties, mechanical properties, transparency, hue and heat resistance. BACKGROUND ART When high-molecular weight materials are used for food packaging materials, these materials are desired to have low gas permeability in order to prevent change of properties of the contents. Of polyester resins, polyethylene terephthalate has been frequently employed for food packaging materials such as various beverage containers because it has an excellent balance between moldability, mechanical properties and gas barrier properties. However, even the polyethylene terephthalate resin is not always satisfactory in the gas barrier properties especially for food packaging materials requiring long-term storage properties. In order to improve the gas barrier properties, studies of polyesters in which a hydroxy carboxylic acid is copolymerized, for example, polyglycolic acid, have been made. In U.S. Patent No. 4,565,851, improvement of gas barrier properties by blending polyethylene terephthalate with polyglycolic acid is disclosed. Polyethylene terephthalate, however, has poor compatibility with polyglycolic acid. Therefore, it is difficult to obtain a transparent resin composition, and it is difficult to obtain a packaging material having excellent appearance. In Japanese Patent Publication No. 21107/1995, a blend of a polyester that is obtained by copolymerizing a hydroxy carboxylic acid and an aromatic dicarboxylic acid with a polyethylene terephthalate component is used, but the polyethylene isophthalate containing a hydroxy carboxylic acid does not contain the hydroxy carboxylic acid in a sufficiently large amount, so that improvement of gas barrier properties of the resin composition is not satisfactory. In Japanese Patent Laid-Open Publication No. 215319/1984, polyethylene terephthalate in which a hydroxy carboxylic acid is copolymerized is disclosed. This polyester is improved in the gas barrier properties of the polyethylene terephthalate, but in order to prepare the polyester, long time polycondensation under the conditions of high temperature and reduced pressure is necessary, so that it is difficult to copolymerize the hydroxy carboxylic acid in a high concentration, and besides, there resides a problem that the resulting polyester has poor hue and low heat resistance. OBJECT OF THE INVENTION It is an object of the present invention to provide a polyester resin composition improved in the gas barrier properties with maintaining excellent mechanical properties, hue, particularly transparency, and heat resistance which are almost on a level with crystalline polyesters. DISCLOSURE OF THE INVENTION There is provided by the present invention a polyester resin composition containing a copolyester having hydroxy carboxylic acid units as constituent units or a polyoxycarboxylic acid, wherein hydroxy carboxylic acid units of 5 or less carbon atoms are contained in amounts of 2 to 75% by mol based on 100% by mol of all the constituent units contained in the composition, and a molar ratio SAA of hydroxy carboxylic acid units both of whose neighboring units are hydroxy carboxylic acid units to all the hydroxy carboxylic acid units contained and a molar ratio Sm of hydroxy carboxylic acid units neither of whose neighboring units is a hydroxy carboxylic acid unit to all the hydroxy carboxylic acid units contained satisfy the following formula: 0 . 03a-Pb/ (l-a) (2Pb+Pa) (formula 2) wherein §a denotes a volume fraction of the polyester resin (A). The right-hand member of the formula 1 is a formula (Maxwell's formula, L.M. Robeson, et al., Die Angew. Makromol. Chem. 29/30, 47(1873)) to numerically determine a gas permeability constant of a two-component model in which a spherical component (A) is dispersed in a component (B) that is a matrix phase. The gas permeability constant PC of the polyester resin composition, said constant being in the range of the invention, is smaller than a value estimated from the Maxwell's formula, and therefore, the component (A) and the component (B) are moderately transesterified to take a form of a block copolymer. Consequently, it can be thought that the gas barrier properties of the polyester resin composition of the invention are improved as compared with those of a simple mixture. The carbonic acid gas permeability constant of the polyester resin composition of the invention is a value measured in the following manner. A given amount of the polyester resin composition having been sufficiently vacuum dried is interposed between two brass plates, aluminum plates and release films, then melted at 280°C, compressed at 10 MPa for 1 minute and then compressed again at 10 MPa with cooling by a compression molding machine set at 0°C to obtain a pressed film having a thickness of 50 to 100 |im. Then, using a carbonic acid gas of atmospheric pressure as a measuring gas, a gas permeability constant of the film is measured at 25°C by means of a gas permeability tester, for example, a G.L. Science GPM-250 device. The polyester resin composition of the invention has a haze of preferably not more than 20, more preferably not more than 5, and a hue (b value) of preferably not more than 15, more preferably not more than 10. The haze of the polyester resin composition of the invention is a value measured in the following manner. A given amount of the polyester resin composition having been sufficiently vacuum dried is interposed between two brass plates, aluminum plates and release films, then melted at 280°C, compressed at 10 MPa for 1 minute and then compressed again at 10 MPa with cooling by a compression molding machine set at 0°C to obtain a pressed sheet having a thickness of about 200 j^m. Then, a haze of the sheet is measured in accordance with JIS K-7105. The hue (b value) of the polyester resin composition is a value measured in the following manner. The above-obtained sheet having a thickness of about 200 |im is fixed on a Teflon (registered trademark) sheet having a thickness of 2 mm to obtain a reflection spectrum, and hue obtained from the reflection spectrum is measured by means of a hue meter, for example, a Minolta Camera spectrocolorimeter CM-1000 model. Preparation of polyester resin composition The polyester resin composition of the invention can be obtained by melt mixing (A) the hydroxy carboxylic acid copolyester or the polyoxycarboxylic acid with (B) the crystalline polyester. After the melt mixing, solid phase polymerization may be further carried out. The temperature for the melt mixing is not specifically restricted provided that it is not lower than a flow temperature of the hydroxy carboxylic acid copolyester or the polyoxycarboxylic acid (A) and not lower than a melting point of the crystalline polyester (B), but the temperature is desired to be in the range of 180 to 300°C, preferably 220 to 290°C. The melt mixing time is in the range of preferably 30 seconds to 4 hours, more preferably I minute to 2 hours. Examples of apparatuses to carry out the melt mixing include a single-screw extruder, a twin-screw extruder, a plastomill, a kneader, and a reactor equipped with a stirring device and a pressure-reducing device. It is desirable to carry out the melt mixing in an inert gas atmosphere and/or under reduced pressure. Of the above apparatuses, a twin-screw extruder having a device capable of freely changing a feed rate is preferably used for the melt mixing. When such an apparatus is used, a feed rate can be controlled, and thereby the melt mixing time of the polyester resin composition can be controlled. Hence, a polyester resin composition having an optimum SAA/SBB value can be continuously obtained. The mixing of the hydroxy carboxylic acid copolyester or the polyoxycarboxylic acid (A) with the crystalline polyester (B) may be carried out in the presence of a catalyst or a stabilizer. The catalyst or the stabilizer may be added in advance to the hydroxy carboxylic acid copolyester or the polyoxycarboxylic acid (A) or the crystalline polyester (B) or may be added during the melt mixing process. Examples of the catalysts include alkali metals; alkaline earth metals,-metals, such as manganese, zinc, tin, cobalt, titanium, antimony and germanium,- and organic or inorganic compounds containing these metals. Examples of the stabilizers and coloring inhibitors include phosphorus compounds and hindered phenol compounds. Of the above compounds, phosphorus compounds are particularly preferable. Examples of the phosphorus compounds include inorganic phosphorus compounds, such as phosphoric acid, phosphorous acid and polyphosphoric acid; phosphoric acid ester compounds, such as trimethyl phosphate and diphenyl phosphate,- and phosphorous acid ester compounds, such as triphenyl phosphite and tris(2,4-di-t-butylphenyl)phosphite. When such a phosphorus compound is contained, the resin composition is excellent in at least hue. In the melt mixing of the hydroxy carboxylic acid copolyester or the polyoxycarboxylic acid (A) with the crystalline polyester (B), a coupling agent having reactivity to both the polyesters may be used in an appropriate amount. The coupling agent is a compound having two or more groups having reactivity to a hydroxyl group or a carboxyl group at the end of the polyester. Examples of the groups having reactivity to a hydroxyl group or a carboxyl group at the end of the polyester include acid anhydride group, isocyanate group, epoxy group, oxazoline group and carbodiimide group. Examples of the compounds having these groups include pyromellitic anhydride, tolylene diisocyanate, hexamethylene diisocyanate, xylylene diisocyanate, diphenylmethane diisocyanate, ethylene glycol diglycidyl ether, resorcinol diglycidyl ether and bisoxazoline. The melt mixing conditions (melting conditions, such as melting temperature, melting time and melt mixing apparatus, mixing conditions, etc.) to obtain the prescribed SAA/SBB value are appropriately determined depending upon the mixing ratio between the hydroxy carboxylic acid copolyester or the polyoxycarboxylic acid (A) and the crystalline polyester (B), compositions thereof, molecular weights thereof, and presence of a catalyst, a stabilizer and a coupling agent. For example, when polyethylene terephthalate having IV of 0.8 dl/g and a hydroxy carboxylic acid copolyester or a polyoxycarboxylic acid composed of a glycolic acid/isophthalic acid/ethylene glycol copolymer (glycolic acid content: 70% by mol) having IV of 0.8 dl/g are mixed in a weight ratio of 90:10 by the use of an apparatus for performing melt mixing at atmospheric pressure, such as a labo-plastomill or a twin-screw extruder, the melt mixing is desirably carried out at a temperature of 280°C for a period of 5 minutes to 15 minutes. When a polyester having a high copolymerization ratio of a hydroxy carboxylic acid, e.g., polyglycolic acid, is used, the melt mixing is desirably carried out for a longer period of time. When the molecular weight of the hydroxy carboxylic acid copolyester or the polyoxycarboxylic acid (A) and/or the crystalline polyester (B) is higher than the above value, the melt mixing is desirably carried out for a longer period of time. When a catalyst is absent or deactivated, the melt mixing is desirably carried out for a longer period of time. Also when a stabilizer such as a phosphoric acid ester is present, the melt mixing is desirably carried out for a longer period of time. Under stronger kneading conditions, the melt mixing is desirably carried out for a shorter period of time. In order to efficiently obtain a polyester resin composition having a SAA/SBB value defined by the present invention for a shorter period of time, it is desirable to select the polyester resins (A) and (B) which are more reactive to each other. For example, when polyethylene terephthalate is used as the component (B), it is preferable to select, as the component (A), a hydroxy carboxylic acid copolymer wherein an aromatic dicarboxylic acid group, such as an isophthalic acid group or a 2,6-naphthalenedicarboxylic acid group, is copolymerized. The polyester resin composition obtained by the melt mixing may be maintained at a temperature of not higher than its melting point for a period of 20 minutes to 400 hours under reduced pressure or in a stream of an inert gas to perform solid phase polymerization. For the solid phase polymerization, a publicly known process is adoptable. For example, pellets, flakes or a powder of the polyester resin composition is maintained in the temperature range of 80°C to a temperature lower by 30°C than the melting peak temperature for a period of 1 to 300 minutes in an inert gas atmosphere to perform precrystallization and then maintained in the temperature range of 130°C to a temperature lower by 10°C than the melting peak temperature for a period of 20 minutes to 400 hours, preferably 1 hour to 100 hours, more preferably 2 hours to 50 hours, to perform solid phase polymerization. The resin composition having been subjected to solid phase polymerization has an increased molecular weight to contribute to improvement of mechanical strength and has a decreased content of a low- molecular weight component. Therefore, it is preferable to carry out solid phase polymerization. By carrying out the solid phase polymerization, further, transesterification reaction is promoted. Hence, even in case of a resin composition prepared by melt mixing for a shorter period of time than the aforesaid preferred period of time and having a SAA/SBB value deviating from the range of the invention, the reaction can be promoted by performing the solid phase polymerization to obtain a SAA/SBB value in the range of the invention, whereby the properties of the resin composition can be enhanced. By performing melt mixing or by performing melt mixing and then solid phase polymerization as described above, the polyester resin composition can be readily imparted with gas barrier properties, etc., with keeping mechanical strength, heat: resistance and transparency of the crystalline polyester. Moreover, a polyester resin composition having higher heat resistance and better hue than a copolyester having the same composition but having been random polymerized too much can be obtained. EXAMPLES The present invention is further described with reference to the following examples, but it should be construed that the invention is in no way limited to those examples. Methods for measuring properties and indications in the present invention are as follows. (I) Composition Composition of a polyester resin was determined by measuring monomer units contained in the polyester resin by NMR spectroscopy. A method to determine the composition is given below taking the later-described Preparation Example 1 as an example. Composition of the hydroxy carboxylic acid copolyester was determined by measuring a 270 MHz proton nuclear magnetic resonance spectrum of a solution of the polyester in deuterated chloroform. Assignments of signals are as follows. 53.5-4.0 ppm (0.67H, ether oxygen neighboring methylene of diethylene glycol unit) 54.1-5.1 ppm (13.38H, methylene of glycolic acid unit, methylene of ethylene glycol unit, and ether oxygen non-neighboring methylene of diethylene glycol unit) 57.4-8.8 ppm (4.OH, cyclic proton of isophthalic acid unit) The monomer proportions were calculated in the following manner from integrated intensity ratios of signals. DEG = 0.67/4 =0.17 unit IA = 4.0/4 =1.0 unit EG = IA-DEG = 1.0-0.17 = 0.83 unit GA = (13.38-4EG-4DEG)/2 = (13.38-3.33 - 0.67)/2 = 4.69 unit Consequently, the following is given. GA/IA/EG/DEG = 4.69/1.0/0.83/0.17 unit = 70.1/15.0/12.5/2.5 mol% Also in Preparation Example 2, Preparation Example 3 and Preparation Example 5, assignments of signals are the same as above, and the monomer proportions were determined in the same manner as described above. Constitution (composition) of the polyester resin composition was anticipated from the compositions of the mixed polyester resins with some exceptions. (2) SAA/SBB A SAA/ABB value was determined as an intensity ratio of a signal showing a bonding pattern wherein both of neighboring units of a hydroxy carboxylic acid unit are hydroxy carboxylic acid units in a NMR spectrum to a signal showing a bonding pattern wherein neither of neighboring units of a hydroxy carboxylic acid unit is a hydroxy carboxylic acid unit in a NMR spectrum. In case of a polyester resin composition obtained from a copolyester (A) constituted of GA, IA, EG and DEG and a crystalline polyester (B) constituted of TA, EG and DEG, a 61-62 ppm region, in which central carbon of glycolic acid is observed, is noted in a 13C-NMR spectrum of the polyester resin composition as measured in a mixed solvent of deuterated chloroform and deuterated trifluoroacetic acid. In this region, a signal of the same glycolic acid carbon is split according to the neighboring units, and there are observed broadly split four kinds, namely, 61.27 ppm, 61.40 ppm, 61.53 ppm and 61.72 ppm. These are assigned to (1) GA-GA-GA, (2) EG(and DEG)-GA-GA, (3) GA-GA-IA or GA-GA-TA, and (4) EG(and DEG)-GA-IA or EG(and DEG)-GA-TA, respectively. The SAA/SBB value was determined as an intensity ratio of the signal (1) to the signal (4). Also regarding a polyester resin composition having different constitution (composition), the SAA/SBB value can be determined in the same manner as described above. (3) Reduced viscosity A reduced viscosity IV of a polyester or a polyester resin composition was measured in a mixed solvent of phenol/tetrachloroethane (1/1 by weight) at 25°C. (4) Melting peak temperature A melting peak temperature of a polyester resin composition was measured by the use of a differential scanning calorimeter DSC-7 model (manufactured by Perkinelmer). Specifically, from a resin composition sufficiently dried in advance, 10 mg of a sample was weighed into a sample pan, heated up to 280°C from room temperature in a nitrogen, atmosphere at a heating rate of 320°C/min, maintained at 280°C for 5 minutes, rapidly cooled down to 20°C at a cooling rate of 320°C/min, maintained at 20°C for 10 minutes and then heated up to 280°C at a heating rate of 10°C/min, and in this process, measurement was carried out. By the use of analysis software attached to the device, a melting peak temperature was determined. (5) Gas barrier properties (5-1) Carbonic acid gas permeability constant Using a pressed film having a thickness of 50 to 100 (im obtained by rapidly cooling a resin or a resin composition in a molten state to not higher than 0°C, a carbonic acid gas permeability constant was measured at 25°C by means of a G.L. Science GPM-250 device. (5-2) (Pb+Pa-F)/(1+F) A density of each component was measured by a density gradient tube (23°C). Using the measured density, a volume fraction t))a of the polyester resin (A) was determined, and from the volume fraction and the carbonic acid gas permeability constants Pa and Pb, a (Pb+Pa-F)/(1+F) value was calculated. In Preparation Example 1, the density of (Al) was 1420 kg/m3; in Preparation Example 2, the density of (A2) was 1407 kg/m3; in Preparation Example 3, the density of (A3) was 1501 kg/m3; in Preparation Example 4, the density of (PGA) was 1590 kg/m3; and in Preparation Example 5, the density of (A4) was 1369 kg/m3 and the density of (B)PET was 1339 kg/m3. (6) Transparency Using a pressed film having a thickness of 200 (im obtained by rapidly cooling a polyester resin composition in a molten state to not higher than 0°C, transparency was measured at 23°C by means of a Nippon Denshoku haze meter. (7) Hue A pressed film having a thickness of 200 urn was fixed on a Teflon (registered trademark) sheet having a thickness of 2 mm, and hue (b value) of the film was measured by means of a Minolta Camera spectrocolorimeter CM-1000 model. Preparation Example 1 In a reaction vessel, 376.2 g (4.95 mol) of glycolic acid, 111.0 g (0.70 mol) of isophthalic acid and 95.4 g (1.54 mol) of ethylene glycol were placed, and they were subjected to esterification reaction at 130 to 200°C in a nitrogen atmosphere at atmospheric pressure for about 13 hours until the reaction product became transparent, with stirring and distilling off water produced. The resulting polyester oligomer was introduced into a glass reactor equipped with a stirring device and a distilling tube. The distilling tube is connected to a vacuum device consisting of a vacuum pump and a reduced pressure controller and has a structure capable of distilling off evaporated matters. To the system, 2.10 g of a germanium type catalyst (germanium dioxide content: 6.7% by weight) was added. The reaction was carried out at 200°C for about 30 minutes in a stream of nitrogen with stirring, then the system was heated up to 220°C over a period of about 4 hours, and the temperature was maintained at 220°C until the reaction was completed. Simultaneously with the beginning of heating, the pressure of the system was reduced to about 0.8 Torr over a period of about I hour, and thereafter, the pressure was maintained at about 0.8 to 0.5 Torr. The reaction was carried out for about 11.5 hours from the beginning of the pressure reduction, and ethylene glycol and the like produced were distilled off from the system. During the polycondensation reaction, viscosity of the reaction product increased with time, and a copolyester (Al) was obtained. The copolyester (Al) thus obtained had a reduced viscosity IV of 0.829 dl/g. The proportions of constituent units of glycolic acid, isophthalic acid, ethylene glycol and diethylene glycol in the copolyester (Al) were 70.1% by mol, 15.0% by mol, 12.5% by mol and 2.5% by mol, respectively. The copolyester was dried at about 40°C for about 20 hours under reduced pressure, and a given amount of the polyester was interposed between two brass plates, aluminum plates and release films, then melted at 200°C, compressed at 10 MPa for 1 minute and then compressed again at 10 MPa with cooling by a compression molding machine set at 20°C to obtain a pressed film having a thickness of about 70 urn. Then, gas barrier properties of the film were measured. As a result, the carbonic acid gas permeability constant was 0.74 cm3 -mm/(m2/day-atom) . Preparation Example 2 In a similar manner to that of Preparation Example 1, 250.0 g (3.29 mol) of glycolic acid, 136.5 g (0.82 mol) of isophthalic acid and 117.3 g (1.89 mol) of ethylene glycol were placed, and they were subjected to esterification reaction in a prescribed manner (for 9 hours). Thereafter, 1.82 g of a germanium type catalyst (germanium dioxide content: 6.7% by weight) was added, and the reaction was carried out for 9.5 hours in a prescribed manner to obtain a copolyester (A2). The proportions of constituent units of glycolic acid, isophthalic acid, ethylene glycol and diethylene glycol in the copolyester (A2) were 58.9% by mol, 20.5% by mol, 16.1% by mol and 4.4% by mol, respectively. Then, gas barrier properties of the copolyester (2) were measured in the same manner as in Preparation Example 1. As a result, the carbonic acid gas permeability constant was 1.1 cm3 -mm/ (m2/day-atom) . Preparation Example 3 In a similar manner to that of Preparation Example 1, 1490 g (19.6 mol) of glycolic acid, 33 g (0.2 mol) of isophthalic acid and 16 g (0.26 mol) of ethylene glycol were placed, and they were subjected to esterification reaction in a prescribed manner (for 9 hours). Thereafter, 8.8 g of a germanium type catalyst (germanium dioxide content: 6.7% by weight) was added, and the reaction was carried out for 5 hours in a prescribed manner. The proportions-of constituent units of glycolic acid, isophthalic acid, ethylene glycol and diethylene glycol in the copolyester (A3) were 98.0% by mol, 1.0% by mol, 0.9% by mol and 0.1% by mol, respectively. Then, gas barrier properties of the copolyester (3) were measured in the same manner as in Preparation Example 1. As a result, the carbonic acid gas permeability constant was 0.15 cm3-mm/(m2/day-atom) . Preparation Example 4 In a glass reactor equipped with a stirring device and a distilling tube, a chloroform solution containing 120 g of glycolide (available from Boehringer Ingelheim Co.) and 72 mg of lauryl alcohol and a chloroform solution containing 36 mg of tin chloride were placed, and the reactor was thoroughly purged with a nitrogen gas Thereafter, the system was stirred and heated at 180°C and atmospheric pressure. In about 1 hour, the contents of the system solidified, so that stirring was stopped, and then heating was continued for 1 hour. Thereafter, the system was heated to 250°C to melt the solid. Thus, a polyglycolic acid (PGA) was obtained. Then, gas barrier properties of the polyglycolic acid (PGA) were measured in the same manner as in Preparation Example 1. As a result, the carbonic acid gas permeability constant was 0.1 cm3-mm/(m2/day-atom) . Preparation Example 5 In a similar manner to that of Preparation Example 1, 152 g (2 mol) of glycolic acid, 166 g (1 mol) of isophthalic acid and 279g (4.5 mol) of ethylene glycol were placed, and they were subjected to esterification reaction in a prescribed manner (for 12 hours). Thereafter, 1.5 g of a germanium type catalyst (germanium dioxide content: 6.7% by weight) was added, and the reaction was carried out for 8 hours in a prescribed manner. The proportions of constituent units of glycolic acid, isophthalic acid, ethylene glycol and diethylene glycol in the copolyester (A4) were 19.7% by mol, 40.2% by mol, 33.9% by mol and 6.2% by mol, respectively. Then, gas barrier properties of the copolyester (4) were measured in the same manner as in Preparation Example 1. As a result, the carbonic acid gas permeability constant was 2.2 cm3 -mm/ (m2/day-atom) . Example 1 90 Parts by weight of commercially available polyethylene terephthalate (Tm: 252°C, [r\] : 0.82 dl/g) having been sufficiently dried by a vacuum dryer and 10 parts by weight of the copolyester (Al) prepared in Preparation Example 1 were mixed by a tapered twin-screw extruder of 30 to 20 mm diameter (manufactured by Haake) at a cylinder temperature, of 280°C, and a feed rate was controlled so as to maintain the mixture in a transparent state. Thus, a copolyester composition was obtained. Then, a melting peak temperature of the copolyester composition was measured. The result is set forth in Table 1. In a 13C-NMR spectrum of the copolyester composition as measured in a mixed solvent of deuterated chloroform and deuterated trifluoroacetic acid, a signal of 61.27 ppm was taken as a signal of GA-GA-GA, and a signal of 61.72 ppm was taken as a signal of EG(and DEG)-GA-IA and EG(and DEG)-GA-TA, and from their intensity ratio, a SAA/SBB value was determined. The copolyester composition was dried at about 70°C for about 20 hours under reduced pressure, and a given amount of the composition was interposed between two brass plates, aluminum plates and release films, then melted at 280°C, compressed at 10 MPa for 1 minute and then compressed again at 10 MPa with cooling by a compression molding machine set at 0°C to obtain a pressed film having a thickness of about 70 (am. Then, gas barrier properties of the film were measured. Further, a pressed film having a thickness of 200 |im was prepared, and haze and hue (b value) of the film were measured. The results are set forth in Table 1. Example 2 In a similar manner to that of Example 1, 90 parts by weight of polyethylene terephthalate and 10 parts by weight of the copolyester (A2) prepared in Preparation Example 2 were mixed to obtain a resin composition. Then, molding and evaluation were carried out in the same manner as in Example 1. The results are set forth in Table 1. Example 3 90 Parts by weight of commercially available polyethylene terephthalate having been sufficiently dried by a vacuum dryer and 10 parts by weight of the copolyester (Al) prepared in Preparation Example 1 were melt mixed by a labo-plastomill (manufactured by Toyo Seiki) for 18 minutes under the conditions of 280°C and 100 rpm to obtain a copolyester composition. Then, molding and evaluation were carried out in the same manner as in Example 1. The results are set forth in Table 1. Example 4 In a similar manner to that of Example 3, 95 parts by weight of polyethylene terephthalate and 5 parts by weight of the polyglycolic acid (Tg: 43°C, Tm: 223°C, number-average molecular weight in terms of PMMA: 100000 (hexafluoro-2-propanol solvent)) prepared in Preparation Example 4 were mixed to obtain a copolyester composition. Then, molding and evaluation were carried out in the same manner as in Example 1. The results are set forth in Table 1. Example 5 93 Parts by weight of commercially available polyethylene terephthalate having been sufficiently dried by a vacuum dryer and 7 parts by weight of the copolyester (A3) prepared in Preparation Example 3 were melt mixed by a labo-plastomill for 10 minutes under the conditions of 280°C and 100 rpm to obtain a copolyester composition. Then, molding and evaluation were carried out in the same manner as in Example 1. The results are set forth in Example 6 90 Parts by weight of commercially available polyethylene terephthalate having been sufficiently dried by a vacuum dryer and 10 parts by weight of the copolyester (A3) prepared in Preparation Example 3 were melt mixed by a labo-plastomill for 18 minutes under the conditions of 280°C and 100 rpm to obtain a copolyester composition. Then, molding and evaluation were carried out in the same manner as in Example 1. The results are set forth in Table 1. Example 7 70 Parts by weight of commercially available polyethylene terephthalate having been sufficiently dried by a vacuum dryer and 30 parts by weight of the copolyester (A3) prepared in Preparation Example 3 were melt mixed by a twin-screw extruder of 30 mm diameter equipped with a constant rate feeder (manufactured by Plastic Kogyo Kenkyusho) under the conditions of a cylinder temperature 280°C, a screw rotation number of 300 rpm and an extrusion rate of 30 g/min to obtain a copolyester composition. Then, molding and evaluation were carried out in the same manner as in Example 1. The results are set forth in Table 1. Example 8 The copolyester composition obtained in Example 7 was sufficiently dried by a vacuum drier, maintained in an oven at 150°C for 2 hours in a stream of nitrogen to perform precrystallization and then maintained at 200°C for 48 hours in a stream of nitrogen to perform solid phase polymerization reaction. Thus, a copolyester composition was obtained. A loss in weight due to the solid phase polymerization was 6% by weight. As a result of NMR spectroscopy, the glycolic acid content was 24.6% by weight (34.1% by mol). Then, the copolyester composition was subjected to molding and evaluation in the same manner as in Example 1. The results are set forth in Table 1. Comparative Example 1 In a similar manner to that of Example 3, 90 parts by weight of polyethylene terephthalate and 10 parts by weight of the polyglycolic acid prepared in Preparation Example 4 were mixed to obtain a copolyester composition. Then, molding and evaluation were carried out in the same manner as in Example 1. The results are set forth in Table 1. The composition had a higher SAA/SBB value and a higher haze value as compared with Example 4. Comparative Example 2 In a reactor equipped with a stirring device and a distilling tube, 16.7 g of ethyl glycolate, 93.2 g of dimethyl terephthalate, 59.6 g of ethylene glycol and 0.08 g of manganese acetate tetrahydride were placed. The reactor was thoroughly purged with nitrogen, then the system was heated up to 220°C from 160°C over a period of 6 hours in a nitrogen atmosphere at atmospheri pressure with stirring, and 1.58 g of an ethylene glycol solution containing 0.12 g of an ester was added with distilling off methanol. The system was thoroughly purged with nitrogen and then stirred at 220°C for 20 minutes in a stream of nitrogen at atmospheric pressure. Subsequently, the system was heated up to 260°C over a period of 80 minutes and maintained at 260°C for 30 minutes. Thereafter, the vacuum pump was actuated to reduce the pressure to 1 Torr over a period of 1 hour, and the system was heated up to 280°C and continuously stirred for 4 hours under reduced pressure of 1 Torr to perform polycondensation. After the polycondensation reaction, a nitrogen gas was introduced into the system to return the pressure of the system to atmospheric pressure, and a copolyester produced was taken out of the reactor. The resulting copolyester had a brown color. Then, the polyester was subjected to molding and evaluation in the same manner as in Example 1. The results are set forth in Table 1. As a result of NMR spectroscopy, the copolyester contained 4.6% by weight (7.5% by mol) of glycolic acid units, and this copolyester had composition near to that of Example 4. This polyester, however, had an SAA/SBB value of 0 and low degree of block polymerization, and had a lower melting peak temperature and lower heat resistance as compared with Example 4. Moreover, because the copolyester was prepared by direct polymerization and had long-time heat history, it had bad hue. Comparative Example 3 90 Parts by weight of commercially available polyethylene terephthalate having been sufficiently dried by a vacuum dryer and 10 parts by weight of the copolyester (A4) prepared in Preparation Example 5 were melt mixed by a labo-plastomill (manufactured by Toyo Seiki) for 10 minutes under the conditions of 280°C and 100 rpm to obtain a copolyester composition. Then, molding and evaluation were carried out in the same manner as in Example 1. The results are set forth in Table 1. The glycolic acid content in the copolyester (4) was low, and the SAA/SBB value was low. Therefore, the gas barrier properties of the copolyester composition were low in spite that the same amount of the hydroxy carboxylic acid copolyester as that in Example 1, Example 2, Example 3 and Example 6 was added to polyethylene terephthalate. Comparative Example 4 Commercially available polyethylene terephthalate was subjected to molding and evaluation in the same manner as in Example 1. The results are set forth in Table 1. (Table Remove) WE CLAIM: 1. A polyester resin composition containing a copolyester or a polyoxycarboxylic acid comprising hydroxy carboxylic acid units as constituent units, wherein hydroxy carboxylic acid units of 5 or less carbon atoms are contained in amounts of 2 to 75% by mol based on 100% by mol of all the constituent units contained in the composition, wherein hydroxy carboxylic acid units of 5 or less carbon atoms, aromatic dicarboxylic acid units and diol units of 4 or less carbon atoms are contained in the total amounts of not less than 95% by mol, based on 100% by mol of all the constituent units of the hydroxy carboxylic acid copolyester or the polyoxycarboxylic acid in the composition and a molar ratio SAA of hydroxy carboxylic acid units both of whose neighboring units are hydroxy carboxylic acid units to all the hydroxy carboxylic acid units contained and a molar ratio SBB of hydroxy carboxylic acid units neither of whose neighboring unit is a hydroxy carboxylic acid unit to. all the hydroxy carboxylic acid units contained satisfy the following formula: 0. 03